Dejagah made his professional debut for Hertha BSC against VfL Bochum in the opening match of the 2004–05 Bundesliga season, playing the last five minutes in a 2–2 home draw in his only first team appearance that year, thus becoming the youngest player to ever play for the club since it was founded in 1892. In his three years at Hertha, he also played in the regional league. Despite starting out as a central defender, he was quickly moved to the forward position and finished as the team's top scorer in 2005–06, with twelve goals in 23 appearances. Hertha's manager, Falko Götz, showed continuous faith in Dejagah by gradually giving the youngster playing time in the Bundesliga, as well as three UEFA Cup showdowns in 2005–06, playing the following season in the Intertoto Cup against FC Moscow.[5]

In 2007, Dejagah joined VfL Wolfsburg. In his first season at Wolfsburg, he showed himself as a talent by scoring eight goals and in his second season, he was a regular part of the Bundesliga winning team.[6] In the 2009–10 season, after the selection of Armin Veh as club manager, he lost his place in the starting line up and he could not return to the starting team under Steve McClaren. With the return of Felix Magath, he found his form and in the 2011–12 season, he showed himself as a key player by scoring and assisting several goals for his club.[7] After Dejagah stated a desire to join Fulham FC in order to fulfill his childhood dream of playing in the Premier League, Magath stated that he wanted Dejagah to stay at Wolfsburg; however, he later sided with the player's wishes to play at Fulham.[8] Dejagah ended his career in the Bundesliga with 19 goals and 25 assists.[9]

In 2004, upon being called up to the German U-19 team, Dejagah scored seven goals in 15 international games, including two against the Netherlands after his team was down 2–0. In 2005, he was invited to play for Germany's U-21.[22]

In October 2007, Dejagah refused to play an under-21 international match for Germany against Israel, in Tel Aviv. He cited "very personal reasons", and said, variously: "There are political reasons. Everyone knows I'm a German-Iranian", "I have more Iranian than German blood in my veins. Besides, I'm doing this out of respect. After all, my parents are Iranian"; and "I have nothing against Israel. But I'm worried about having problems later when traveling to Iran".[3]Bild (Germany's top-selling newspaper), Ronald Pofalla and Friedbert Pflüger (the General Secretary and a leading member of the Christian Democratic Union), and Charlotte Knobloch (President of the Central Council of Jews in Germany) demanded that Dejagah be excluded from Germany's national team.[23][24]

Dejagah said in a Stern interview that he chose not to play in Israel because, since the Iranian Revolution, Iranians traveling to Israel are subjected by the Iranian government to harsh punishments and several years in prison, and he had concern for his parents who travel to Iran and his other relatives who live in Iran. He also said he did not make his choice because of personal political, antisemitic, or racist reasons.[3][25] Since the revolution, the Iranian government has warned Iranian athletes not to travel to Israel or compete against Israeli athletes.[3][26]

Dejagah in a friendly match against Montenegro prior to the 2014 World Cup.

After a meeting with Theo Zwanziger, the president of the German Football Association, and national technical director Matthias Sammer, Dejagah stated that his reason was that he was concerned because of his Iranian origins about the welfare of his family and relatives if he were to play in Israel; therefore, he was not excluded from Germany's national team.[26][27] Zwanziger said, "He clearly stated that his request not to be nominated for the game in Israel had no racist or anti-Semitic background. He credibly assured us that, because of his Iranian origins, he had only been concerned with the well-being of his family and relatives."[26] Zwanziger added that Dejagah was now prepared to play in the home tie against Israel if selected.[28]

Seven years later, in April 2014, Dejagah spoke on the matter, saying, "This was a long time ago; it is in the past. Yes, it helped me grow up but now I only look to the future."[29]

It had been suggested that Dejagah might play for Iran at the international level rather than Germany, but he said he never really considered it an option.[30] Having played for Germany at the age of 21, he was originally no longer eligible to play for Iran,[31] however, FIFA rule changes in July 2009 meant that he could then play for Iran.[32] In late December, Iran's coach Afshin Ghotbi approached Dejagah in regards to playing for Iran at the upcoming 2011 AFC Asian Cup in Qatar, but his playing for Iran was placed on hold as he wanted to pursue first team football with his club VfL Wolfsburg instead.[33]

During his time with Fulham, Dejagah resided in London, England, with his German-Iranian wife and daughter.[29] He wears the Nike Mercurialboot line and has their names embedded onto the external sides of his boots. He has tattoos with Berlin on one arm and Tehran on the other, and the phrase, "Never forget where you're from" on his neck.[2][29]